Hi XC,
my mother and i ride welsh cobs right now and they can be challenging! i know exactly what you are feeling! One of the cobs i ride not only is wide and flat, but has extra skin across her back and has an annoying habit of stepping out sideways with a rear leg. that causes my opposite hip to drop and the whole saddle slides to the side.
my first piece of advice is not to buy a tree less saddle. the best saddles that i have found are a lazer that is has an adjustable tree
http://www.laserequestrian.com/
and a duett
http://www.duettsaddles.com/allpurpose.htm
the saddle fit is important because you don’t want to “pad up” a saddle that is too narrow. but on the haflinger i use a fleece half pad on long hunts, to save his back. or a dressage saddle! on one really back flat cob i use a leather/felt half pad.
second, your saddle needs to be pretty far back on their back to allow their shoulder to swing. the Duett all purpose saddle flaps aren’t too far forward, andthere is still room for your knee.
so that leads to my 3rd peice of advice. leave your stirrups a bit longer. i feel like i’m “perched” up there, without any grip if the saddle starts to roll.
also, very important on a saddle is the length of the billets. if the billets are longer and the girth is short, then the saddle has a better chance of staying in place.
and these girths have been a life saver: http://www.profchoice.com/htmldocs/prd/english/index1.html
if you buy one thing, it should be this girth. i’ve had a few galls by putting it too close to their elbow tho. this girth needs to be cleaned after every use, at least rinsed with water.
i also use a breastcollar for hanging on! and for mounting, i grab some mane and the opposite side of the breastcollar and jump as high as my big rear will go! even though the girth is tight, the saddle will slip when i mount.
as for training, the cobs and haflinger can be quick to turn and stop as you know, so i wear spurs. when they do quick turns or stops, i spur them. they quit doing that after a while and smooth out.
but i’ve never been able to stay on a bucking cob! i always get thrown out of the saddle…but maybe it’s me…it’s probably me…
i hope that helps!!
do share your experiences too, i’m always looking for something to try on the cobs!